Improving the design of solar cells

Photovoltaic (PV) systems convert the sun’s light directly to electrical power by absorption in a suitable semiconducting material. First generation PV systems predominantly use wafers of silicon as the absorber material. Second generation PV systems use thin films of more efficient absorber materials, such has copper-indium-gallium-diselenide (CIGS) and cadmium-telluride (CdTe). Third generation PV systems will exploit the novel properties of materials at the nano scale.

Although the raw materials for second and third generation PV are considerably more expensive than silicon, they absorb so much more light that much smaller quantities are needed. Importantly, they can be deposited on inexpensive substrates, such as metal foils and glass, via ink jet printers or electroplating, enabling novel device structures with much lower fabrication costs than traditional silicon fabrication methods.

To better understand the metrological needs for manufacturing these novel devices, researchers in the Metallurgy Division of the Material Measurement Laboratory have begun electrodepositing CdTe with the goal of producing 2D and 3D micro- and nano-structures. Experiments are underway to determine the conditions (what must industry measure and control?) for deposition of the different required dopant compositions from a single electrochemical bath. Subsequently, trench-like PV structures will be deposited on patterned electrodes, developed in collaboration with the Center for Nanoscale Science and Technology.

To guide these experiments and to help interpret their results, the Metallurgy researchers have developed software to simulate these devices. Unlike existing codes, which are either limited to 1D or are proprietary, the Metallurgy code gives the user complete freedom to specify the geometry of the device, the materials used, and the illumination. This new Open Source code is completely configurable by the user and can be readily integrated with simulations of the deposition process and of phase transformations undergone during processing and use. As this code is validated against experiments and against other codes, it will be made freely available to the photovoltaic industry and to academia.